Process for non-oxidative dehydrogenation of alkane

ABSTRACT

The invention relates to a process for producing an alkene by non-oxidative dehydrogenation of an alkane, comprising contacting a feed stream comprising the alkane with a catalyst composition comprising an unsupported catalyst comprising ZrV 2 O 7  at a temperature of 400 to 600° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Application No. 12004056.3,filed May 24, 2012, which is incorporated herein by reference in itsentirety.

The present invention relates to a process for producing an alkene bythe non-oxidative dehydrogenation of an alkane.

Isobutene is produced by the dehydrogenation of isobutane and as aby-product from fluid catalytic cracking (FCC) and steam crackingoperations. The major application of isobutene is as feed stock in themanufacture of Methyl-tert-butyl-ether (MTBE) and production of polymerslike butylrubber, polybutene and isoprene. Chromia/Alumina catalysts aretypically used for the production of isobutene from isobutane.

Disposal of this catalyst poses problem due to environmental regulationsand involves major expenditure, since chromium is a known carcinogen.Chromium exists in two stable oxidation states, namely +3 and +6. Ingeneral, chromium (VI) is more toxic than chromium (III). Long-termexposure to chromium has been associated with lung cancer in workersexposed to levels in air that were 100 to 1,000 times higher than thosefound in the natural environment. One of the safe methods of disposal isto reclaim Cr from the spent catalyst for reuse and then dispose thecatalyst. However, the reclaiming of Cr from the catalyst is notfeasible. Some of the other ways the waste catalyst can be disposed areby use in cement bricks, use in steel industry and in refractoryindustry. To overcome these problems, development of non-chromiacatalyst is essential.

There has been an ongoing research for non-chromium catalyst for theproduction of isobutene by the non-oxidative dehydrogenation ofisobutane, which matches the performance of the chromia catalyst withrespect to conversion, selectivity and catalyst life.

Harlin et. al., Journal of Catalysis 203, 242-252 (2001) describes theuse of vanadium oxide catalyst modified by Zr supported on alumina in anon-oxidative dehydrogenation process of isobutane. The catalyst wasprepared by the incipient wetness impregnation method with alumina asthe support. The catalyst was prepared by applying an aqueous solutionof ZrO(NO₃)₂-7H₂O and an aqueous solution of NH₄VO₃ to alumina particlesand drying and calcining the particles. The catalyst comprises γ-Al₂O₃,ZrO₂ and a minor amount of ZrV₂O₇. The isobutane feed was diluted withnitrogen (N₂:Isobutane =9:1).

There is a continuous need in the art for a process which results in ahigher conversion rate of isobutane and selectivity to isobutene.

Accordingly, the present invention provides a process for producing analkene by non-oxidative dehydrogenation of an alkane, comprisingcontacting a feed stream comprising the alkane with a catalystcomposition comprising an unsupported catalyst comprising ZrV₂O₇ at atemperature of 400 to 600° C.

Surprisingly, the process according to the present invention shows ahigh conversion rate and a high selectivity.

It is noted that Khodakov et. al., Journal of Catalysis, 343-351 (1998)mentions the use of bulk ZrV₂O₇ prepared from a stoichiometric mixtureof V₂O₅ and ZrO₂ for a dehydrogenation process of isobutane. Thedehydrogenation process in this publication is an oxidativedehydrogenation process. Bulk ZrV₂O₇ did not show catalytic effects.This publication also mentions the use of vanadium oxide catalystssupported on zirconia in an oxidative dehydrogenation process ofisobutane.

As used herein, the term “non-oxidative dehydrogenation” is understoodto mean that the dehydrogenation proceeds substantially in the absenceof an oxidizing agent such as oxygen, i.e. the amount of the oxidizingagent in the feed stream is at most 5 vol %.

A supported catalyst is typically prepared by applying a solution of theactive component to a material having a larger surface area and dryingand calcining the material. As used herein, the term ‘unsupportedcatalyst’ is understood to mean that the catalyst is not deposited on amaterial by a process involving calcination. When the catalyst is mixedwith a further material without heating, said further material isunderstood to be not a catalyst support, and the catalyst in theresulting composition remains as an unsupported catalyst.

As used herein, the term “catalyst composition” is understood to mean acomposition consisting of the catalyst (active phase) and any othersuitable components such as a catalyst binder.

Preferably, said alkene is selected from the group consisting ofethylene, propylene, n-butene, isobutene, 1,3-butadiene and mixturesthereof and said alkane is selected from the group consisting of ethane,propane, n-butane, isobutane, and mixtures thereof. Most preferably, theprocess according to the invention is a process for producing isobutenefrom isobutane.

Preferably, the catalyst comprises ZrV₂O₇ as a major phase. Accordingly,the catalyst comprises at least 30 wt % of ZrV₂O₇. More preferably, thecatalyst comprises at least 40 wt %, at least 50 wt %, at least 60 wt %,at least 70 wt %, at least 80 wt %, at least 90 wt %, more preferably atleast 95 wt %, even more preferably at least 99 wt %, of ZrV₂O₇.

Preferably, the feed stream comprises at least 80 vol % of the alkane,preferably at least 90 vol %, more preferably at least 95 vol %, evenmore preferably at least 99 vol %, of the alkane. Accordingly, theprocess is performed with little or no amount of feed diluents. This hasan advantage of eliminating the downstream diluents separation stepwhich lead to the reduction of overall operation cost. The throughputfor a given size of reactor is increased.

Preferably, the contacting step is performed at a temperature of at most560° C., preferably at 540-560° C., around 550° C. This has an advantagethat the required energy for the process is low.

Preferably, the unsupported catalyst used in the process according tothe present invention has been prepared by a process comprising thesteps of: mixing ammonium meta-vanadate and zirconyl nitrate, drying themixture and calcining the dried mixture at a temperature of 600-900° C.for 2-10 hours, preferably 650-850° C. for 4-6 hours, more preferably700-750° C. for 4 h.

Preferably, ammonium meta-vanadate and zirconyl nitrate are mixed at amolar ratio between V and Zr of 0.4:1-1.3:1, preferably 0.5:1-1.1:1,more preferably 0.9:1-1.1:1. This leads to the formation of a higherproportion of ZrV₂O₇ in the resulting catalyst and a better performanceof the catalyst.

The catalyst composition may substantially consist of the catalyst, i.e.the catalyst may be used in the process according to the inventionwithout mixing the catalyst with other components. However, preferably,the catalyst composition used in the process of the invention may beprepared by mixing the catalyst with a catalyst binder, such as Al₂O₃.The mixing of the catalyst with Al₂O₃ is performed without heating, sothat no phase change of Al₂O₃ occurs. The catalyst composition preparedin this way shows a high isobutane conversion rate and a higherisobutene selectivity. Al₂O₃may be α-Al₂O₃ or γ-Al₂O₃. γ-Al₂O₃ is morepreferred since the isobutane conversion rate is higher.

The weight ratio between the catalyst and Al₂O₃ may be between 0.2:1 to5:1, more preferably 0.5:1 to 2:1, more preferably 0.9:1 to 1.1:1.

According to a further aspect of the present invention, a process isprovided for preparing a catalyst composition comprising an unsupportedcatalyst comprising ZrV₂O₇, comprising the steps of: mixing ammoniummeta-vanadate and zirconyl nitrate, drying the mixture and calcining thedried mixture at a temperature range of 600-900 ° C. for 2-10 hours toobtain the unsupported catalyst and mixing the unsupported catalyst withAl₂O₃ without heating.

A further aspect of the present invention provides the catalystcomposition obtainable by the process according to the presentinvention.

It is noted that the term ‘comprising’ does not exclude the presence ofother elements. However, it is also to be understood that a descriptionon a product comprising certain components also discloses a productconsisting of these components. Similarly, it is also to be understoodthat a description on a process comprising certain steps also disclosesa process consisting of these steps.

The present invention is illustrated below by referring to the followingnon-limiting experiments.

EXPERIMENTS Comparative experiment

γ-Al₂O₃ supported catalyst was prepared by incipient wet impregnationmethod Ammonium meta-vanadate and zirconyl nitrate were mixed togetherto form a solution in the presence of deionized water such that themolar ratio between V and Zr were 1:1. 5 g of γ-Al₂O₃ was taken in asilica crucible and placed on the hot plate. The mixed solution ofammonium meta-vanadate and zirconyl nitrate was then added dropwise withstirring until the whole mass was dry. Temperature of the hot plate waskept at a temperature of 100° C. to evaporate excess water. Subsequentlythe dried mass was heated at a temperature of 800° C. for 4 h. Asupported catalyst of ZrV₂O₇ supported on γ-Al₂O₃ was obtained.

The supported catalyst was mixed with γ-Al₂O₃ at 1:1 wt ratio. Themixture was crushed and sieved to a particle size of 250-500 micron. Acatalyst composition comprising a supported catalyst (ZrV₂O₇ supportedon γ-Al₂O₃) and a catalyst binder (γ-Al₂O₃) was obtained.

Isobutane was contacted with the catalyst composition obtained in anon-oxidative atmosphere in the absence of any feed diluents at atemperature of 550° C. The results are shown in Table 1.

Example 1

Ammonium meta-vanadate and zirconyl nitrate were mixed together suchthat the molar ratio between V and Zr were 1:1. During the mixing a veryminimum quantity of water was added. The whole mass was then dried andcalcined at 800° C. for 4 h to obtain a non-supported catalystcomprising ZrV₂O₇.

The non-supported catalyst was mixed with γ-Al₂O₃ at 1:1 wt ratio. Themixture was crushed and sieved to a particle size of 250-500 micron. Acatalyst composition comprising a non-supported catalyst (ZrV₂O₇) and acatalyst binder (γ-Al₂O₃) was obtained.

Isobutane was contacted with the catalyst composition obtained in anon-oxidative atmosphere in the absence of any feed diluents at atemperature of 550° C. The results are shown in Table 1.

Example 2

Example 2 was repeated, except that γ-Al₂O₃ was replaced by α-Al₂O₃. Theresults are shown in Table 1.

Isobutane Isobutene Isobutene conversion selectivity yield Catalystcomposition (%) (%) (%) Comp. Ex 28.9 65.9 19.0 Ex. 1 41 92 37.7 Ex. 237 93 34.4

It can be seen that the non-supported catalyst leads to a higherisobutane conversion, isobutene selectivity and isobutene yield.

It can also be seen that the catalyst composition comprising γ-Al₂O₃leads to a higher isobutane conversion and isobutene yield as comparedto the catalyst composition comprising α-Al₂O₃ while the isobuteneselectivity is comparable.

1. A process for producing an alkene by non-oxidative dehydrogenation ofan alkane, comprising contacting a feed stream comprising the alkanewith a catalyst composition comprising an unsupported catalystcomprising ZrV₂O₇ at a temperature of 400 to 600° C. to produce thealkene.
 2. The process according to claim 1, wherein said alkene isselected from the group consisting of ethylene, propylene, n-butene,isobutene, 1,3-butadiene and mixtures thereof and said alkane isselected from the group consisting of propane, n-butane, isobutane, andmixtures thereof.
 3. The process according to claim 1, wherein thecatalyst comprises at least 30 wt % of ZrV₂O₇.
 4. The process accordingto claim 1, wherein the feed stream comprises at least 80 vol % of thealkane.
 5. The process according to claim 1, wherein the contacting isperformed at a temperature of at most 560° C.
 6. The process accordingto, wherein the unsupported catalyst has been prepared by a processcomprising: mixing ammonium meta-vanadate and zirconyl nitrate, dryingthe mixture; and calcining the dried mixture at a temperature of600-900° C. for 2-10 hours.
 7. The process according to claim 6, whereinammonium meta-vanadate and zirconyl nitrate are mixed at a molar ratiobetween V and Zr of 0.4:1-1.3:1.
 8. The process according to claim 1,wherein the catalyst composition has been prepared by mixing theunsupported catalyst with Al₂O₃ without heating.
 9. The processaccording to claim 8, wherein Al₂O₃ is γ-Al₂O₃.
 10. The processaccording to claim 8, wherein the weight ratio between the catalyst andAl₂O₃ is between 0.2:1 to 5:1.
 11. A process for preparing a catalystcomposition comprising an unsupported catalyst comprising ZrV₂O₇,comprising: mixing ammonium meta-vanadate and zirconyl nitrate; dryingthe mixture and calcining the dried mixture at a temperature range of600-900° C. for 2-10 hours to obtain the unsupported catalyst; andmixing the unsupported catalyst with Al₂O₃ without heating.
 12. Theprocess according to claim 11, wherein ammonium meta-vanadate andzirconyl nitrate are mixed at a molar ratio between V and Zr of0.4:1-1.3:1.
 13. The process according to claim 11, wherein the weightratio between the unsupported catalyst and the catalyst binder isbetween 0.2:1 to 5:1.
 14. The catalyst composition obtainable by theprocess according to claim 11.